Radioactive waste sites exist in the United States that contain large volumes of material, several sites are in excess of 100,000 cubic yards. The number of such sites and concern for their management has significantly increased over recent years because of renewed concern over environmental issues, including the disposal of radioactive waste. The best method for the removal of radioactivity from such sites and their long term disposal requirements concerns many governmental agencies and private industry. The number of these sites that can treat and handle the huge amounts of radioactive waste are limited, due in part to the difficulty in identifying and siting new treatment and disposal facilities.
Usual processing of these radioactive sites requires the treatment of large quantities of material, only a portion of which is in fact usually radioactive. Because of tremendous difficulties in economically treating such massive quantities of material to remove the radioactive portion and also meet the radioactivity level requirements for disposal set by government agencies, the best disposal method employed to date has been burial of the radioactive material. The burial method requires hauling large quantities of material, that are regulated as radioactive waste material, frequently many miles to an approved burial site. Therefore, economical methods for the reduction of the volume of the radioactivity for disposal at these sites have been actively sought.
Several methods for a volume reduction of radioactive waste have been explored in the literature. Examples of review articles that describe the issues are:
Energy Digest 15(4), 10-16 (1986), "World Status of Radioactive Waste Management"; PA1 Karl Heinz et al., Nuclear Engin. and Design 118, 115-122 (1990), "Volume Reduction, Treatment and Recycling of Radioactive Waste"; PA1 "Low-Level Radioactive Waste Reduction and Stabilization Technologies Resource Manual" (December 1988) by Ebasco Services Inc., Bellevue, Wash. for EG&G Idaho, Inc. under subcontract C85-131069 and for the U.S. Department of Energy, Idaho Operations Office under contract DE-AC07-76IDO1570; PA1 A. H. Kibbey and H. W. Godbee, "A State-of-the-Art Report on Low-Level Radioactive Waste Treatment", Oak Ridge National Laboratory, Oak Ridge, Tenn. under the Nuclear Waste Programs ORNL/TM-7427 (1980); and PA1 "Technological Approaches to the Cleanup of Radiologically Contaminated Superfund Sites" by the U. S. Environmental Protection Agency, No. EPA/540/2-88/002 (August 1988). PA1 a) extracting magnesium from a magnesium slag, which slag contains radioactive thorium and its daughters: PA1 b) forming an aqueous magnesium slurry from the magnesium slag and water; PA1 c) reacting the magnesium slurry with carbon dioxide; PA1 d) selectively concentrating the radioactive thorium and its daughters such that the radioactivity is separated from the magnesium; and PA1 e) reducing the volume and/or weight of radioactive solids for disposal as radioactive waste.
When the radioactive component is a solid, then various physical separation techniques have been investigated based on methods involving: screening; classification; gravity concentration: and/or physical separation using flotation. The screening technique separates components on the basis of size and can be used either on dry material or water can be added, the material is separated by passing it through certain size screens. The classification technique is used to separate particles of material based on their settling rate in a liquid. The gravity concentration technique utilizes density differences to separate materials into layers. The flotation technique is based on physical and chemical phenomena as well as particle size differences. One technique based on gravity and particle size differences is taught in U.S. Pat. No. 4,783,253. In general, however, physical separation techniques will not be useful if the radioactive material is distributed uniformly within each particle size throughout all of the components comprising the mixture.
When the radioactive component is in solution, then filtration, carbon treatment, ion exchange, and/or precipitation techniques are often used. Care must be exercised if a person is considering using any one of these techniques since a high degree of selectivity is required. For example, a precipitation technique may concentrate the majority of the radionuclides in a solid matrix, but if the precipitation was not quantitative then the solution from which the precipitation was preformed may still have sufficient radioactivity to be of concern for disposal. Thus if the process is not selective, the total volume of material for disposal after such processing can increase. These concerns have been raised by Raghaven et al. ["Technologies Applicable for the Remediation of Contaminated Soil at Superfund Radiation Sites", U. S. Environ. Prot. Agency Res. Dev., [Rep.] EPA (1989), EPA/600/9-89/072, Int. Conf. New Front. Hazard. Waste Management, 3rd. ed., 59-66 (1989)] where they indicate that of the 25 contaminated Superfund sites discussed that no chemical extraction or physical separation techniques have actually been used in a remediation situation and that their use must be approached with extreme caution.
Some volume reduction techniques involve the use of incinerators and compactors. If incineration is used, then the off-gases and particulates that are produced must be constantly monitored and treated to ensure that radioactivity is not being released to the environment. Supercompactors, which are compactors that can exert forces in excess of 1,000 tons, have been used to achieve even greater reductions in volume. However, these supercompactors represent a very large capital investment.
Volume reductions based on chemical extraction techniques using mineral acids have been reported. For example, U.S. Pat. No. 4,689,178 discloses the use of sulfuric acid in the recovery of magnesium sulfate from a slag containing magnesium and uranium metal and the oxides, fluorides and mixed oxides and fluorides of the metals. The desired outcome is that the radioactivity will occupy less volume than it did in the original slag. A similar process is described in U.S. Pat. No. 2,733,126.
A process for the treatment of Magnox fuel element debris is described by D. Bradbury in "Development of Chemical Methods of Radioactive Waste Management for U. K. Power Reactor Sites", ANS/DOE Treatment & Handling of Radioactive Wastes (Batelle/Springer-Verlag) Conf., Richland, Wash., pp. 377-380 (April 19-22, 1982). Magnox alloy consists essentially of magnesium metal where about 1% of other alloying elements have been added. After irradiation, the levels of long-lived radioisotopes is reported to be low. Minor constituents in the waste debris, for example the approximately 5 G springs that are used with the spent Magnox fuel elements are produced from a nickel alloy that contains small amounts of cobalt. During irradiation the cobalt becomes activated to give cobalt-60 and the resulting radioactivity of the springs is far greater than from the irradiated Magnox. The process to isolate the radioactive debris from the Magnox alloy involves corroding away the magnesium in an aqueous medium. The process is conducted in a batch wise manner with large quantities of rapid flowing fresh water with carbon dioxide sparging. Care must be taken to maintain the magnesium concentration below the solubility limit, hence the large quantities of water. Since the dissolution also produces hydrogen gas with an exothermic reaction, proper handling techniques are required. A typical Magnox batch dissolution would take 20 days. The degree of dissolution of some of the radionuclides associated with the Magnox process is given by Bradbury et al. in "Magnox Dissolution in Carbonated Water. A Method of the Separation and Disposal of Magnox from Fuel Element Debris Waste", Water Chem. 3, 345-352 (1983) BNES, London. For cobalt-60, 29% was dissolved in the effluent.
The above issues have resulted in large increases in cost associated with the disposal of waste [see, for example, "Low-Level Radioactive Waste Regulation", ed. Michael E. Burns, pub. Lewis Publishers, inc. (1988)]. The need therefore to minimize the amount of radioactive waste that has to be placed in an approved landfill or treated in other ways has become of critical importance.
To date no technique exists which is cost effective, safe to the environment and technicians, and attains the selectivity needed for the radioisotopes.